Multimodal Microscopy Using “Half and Half”
Contact Mode and Ultrasonic Force Microscopy
Mark S. Skilbeck , Alex J. Marsden , Gaoxiang Cao , Ian A. Kinloch , Rachel S. Edwards , Neil R. Wilson
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
Email: m.skilbeck@warwick.ac.uk
School of Materials, University of Manchester, Manchester, M13 9PL, UK
1
1
2
2
1
1
1
2
Theory
AM Frequency
AFM Feedback Control
Reference
Lateral
Amplitude
b. Deflection
Lock-in Amplifier
Lock-in Amplifier
c. Lateral
(Friction)
Stiffness
Reference
Sample
d. Current
Topography
AM Frequency
Transducer
A
a. Drive
Function Generator
-
• Left - Schematic of the UFM setup with the additional
contact mode channels also shown.
• Above - signals seen in the labelled channels of the
schematic.
Ultrasound Frequency
»» “Fast” and “slow” refer to the relative tip velocity.
»» Blue lines are for the retrace signals.
Sample Stage
Friction
• No friction during ultrasound period, friction during
contact period.
• Measure lateral deflection of cantilever.
• Affected by topography, remove first order influence
by subtracting the retrace from the trace.2
• Can also measure the lateral amplitude using a lock-in
amplifier, which gives friction and is independent of
topography, allowing single direction scans.
• Example (below) - partial coverage graphene on
copper, all images are acquired simultaneously.
a
4 μm
Conductivity and Fragile Samples
• Conductive contact is broken when ultrasound is on.
• Current still flows during contact period.
• Lack of friction during ultrasound prevents dragging
of surface material - reducing damage to delicate
samples.3
• Example (below) - conductivity map of single walled
carbon nanotubes taken with UFM active.
»» Nanotube network attached to gold contact held at a
bias of 2 V.
»» Repeated scanning was possible while UFM was
active, whilst scanning in contact mode swept
nanotubes away.
4 μm
10-5
10-6
10-7
b
10-8
10-9
Current (A)
a.Topography (600 nm full data scale) - sample is too
rough to clearly resolve graphene flakes.
b.UFM - graphene appears slightly softer than copper
(out of plane stiffness).
c. Friction (later trace minus retrace) - Clear contrast
between graphene and copper.
d.Lateral amplitude (retrace only) - clear contrast, same
as the standard friction image.
a. Drive
Figures
Modulation
Carrier Wave
+
Bias
b. Deflection
• Sample is mounted on piezoelectric transducer and
oscillated at frequencies well above the cantilever
resonance.
• Cantilever is stationary over oscillation period, with
the deflection determined by average force, which
depends on the shape of the force curve (i.e. the
sample stiffness).1
• Stiffer samples cause greater deflection.
• Ultrasound is modulated at lower (<10kHz)
frequencies to be off for half a modulation period,
decoupling topography and stiffness.
• During the off period, the tip is in contact with the
sample, allowing contact mode techniques, such as
friction force microscopy (FFM) and conductive AFM
(cAFM) to be used in conjunction with UFM.
Real
c. Lateral (Slow) c. Lateral (Fast)
• Ultrasonic Force Microscopy (UFM) is used to
investigate relative stiffness of a sample surface.
• Application of ultrasound is modulated at much lower
frequencies, resulting in a “half and half” static and
dynamic imaging mode.
• Contact mode techniques can be applied using the
static portion.
• Multimodal scanning demonstrated using friction and
conductive AFM combined with UFM.
• UFM also reduces damage to samples that is typical
of contact mode, allowing use of these techniques on
delicate samples.
Idealised
d. Current
Summary
Multiple Channels
• Can combine many contact mode techniques with
UFM as required.
• Example (below) - UFM, friction and conductivity of a
graphene in epoxy composite.
a.Topography (150 nm full scale) - sample surface
is very rough, though some surface flakes can be
distinguished.
b.UFM - surface flakes are visible as softer (darker)
regions.
c. Friction (lateral trace minus retrace) - surface flakes
have lower coefficient of friction than the surrounding
epoxy.
d.Current (2 μA full scale), with a 0.5 V bias applied
to top of sample - surface flakes have a high
conductivity. Further detail is also present, showing
the extended subsurface conductivity network
formed by the flakes.
»» Strong correlation between all property channels.
a
b
10-10
10-11
c
1 μm
c
d
d
We acknowledge support from the University of
Warwick through a Chancellor’s scholarship to MSS and
support from the EPSRC through grant EP/J015202/1.
References
1. Yamanaka, K., Ogiso, H., & Kolosov, O., Applied Physics Letters, 64, 178 (1994).
2. Marsden, A. J., Phillips, M., & Wilson, N. R., Nanotechnology, 24, 255704 (2013).
3. Dinelli, F., Castell, M., & Ritchie, D., Philosophical Magazine A, 80, 2299 (2000).
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